In exploring in subsea and other areas underlying a body of water, it is desirable to provide a source of energy for propagating acoustic pulses into the water. Since water is a good conductor of sound, it normally is not necessary to generate pulses near the floor of the waterbody; they can be, and desirably are, produced near the water's surface. These acoustic pulses propagate down through the water and may be reflected, to some extent, from a reflective horizon back to an array of hydrophones, or like equipment, waiting near the surface of the water. The reflective horizon may be the floor of the waterbody or a subfloor geologic formation. Analysis of the signals produced by the hydrophones can provide some instruction concerning the structure of the subfloor geological formation and attendant petroleum accumulation in those formations.
The term "water" as used herein is meant to include swampwater, mud, marshwater and any other liquid containing sufficient water to enable operation of the invention.
The usual technique for marine seismic exploration involves towing one or more seismic sources and one or more seismic cables (or "streamers") behind an exploration vessel. Each streamer contains arrays of hydrophones which detect acoustic energy generated by the seismic source and reflected from subfloor geological formations. The term "hydrophone" as used herein is meant to include any type of instrument for detecting acoustic waves.
The depth to which subfloor geological formations can be detected depends on, among other variables, the ratio of signal strength to noise level at the hydrophones. In many marine regions, multiple reflections between the water-air interface and either the water-floor interface or a shallow subfloor reflecting surface are a significant source of noise. Multiple reflections are particularly serious if they should happen to reinforce each other so as to set up standing waves between the water-air interface (hereinafter referred to as the "water surface") and the other reflecting surface from which the noise reflects (hereinafter referred to as the "floor"), whether it be the water-floor interface or a shallow subfloor reflecting surface. These standing waves or "acoustic reverberations" (occasionally called "ringing") in the water may produce multiple reflections extending almost the entire length of seismograms produced during seismic exploration operations, to a distance beyond that at which it is expected that useful reflection information will be obtained. As a result, the relatively weak reflection events from deeper reflection horizons may be completely obscured by the multiple reflections.
Various schemes have been proposed for reducing such reverberating noise. One such scheme is disclosed in U.S. Pat. No. 3,943,484, issued Mar. 9, 1976 to Balderson. Balderson discloses positioning both the seismic source and hydrophones at a depth chosen so that undesirable reflections from the water surface and the water floor will arrive at the hydrophones substantially 180.degree. out of phase, and will thus cancel. U.S. Pat. No. 2,757,356, issued July 31, 1956 to Haggerty, discloses a similar scheme. Haggerty discloses towing at least two streamers, containing hydrophones, separated by a distance selected so that reverberating acoustic noise in the nature of standing waves detected at one streamer will be substantially 180.degree. out of phase from the noise detected at another streamer, and then combining the signals detected at such streamers to reduce or cancel the reverberating noise. Significant practical difficulties must be overcome to practice the invention of Balderson or that of Haggerty. Both require that the distance between the water surface and the water floor be known in each region to be explored, in order to select the depth or depths at which the hydrophones are to be towed. Such distance may vary substantially in the region to be explored. Both also require that the hydrophones be maintained in position at the selected depth or depths, which selected depth or depths may vary substantially with the position of the seismic vessel as it moves through the region to be explored.
United Kingdom Pat. No. 1,376,513, granted to Imperial Chemical Industries Limited and published Dec. 4, 1974, discloses a scheme for reducing noise at a seismic streamer due to reflections from the water surface of acoustic waves proceeding up toward the water surface from a submerged explosive charge. The U.K. Patent discloses disposing a mantle of gas (such as gas bubbles, closed-cell foamed plastic, or a volume of gas entrapped beneath an inverted vessel) between the explosive charge and the water surface.
U.K. Pat. No. 1,376,513 does not specify the position of the mantle relative to the streamer. In typical marine seismic data gathering operations, the streamer and seismic source will be separated by a substantial horizontal distance, so that a gas mantle disposed above the source will be located far from the region above the streamer. Nor does the U.K. Patent specify any particular size or shape for the mantle beyond noting that where a sheet of foamed plastic is used as the mantle, the sheet may be either flat or curved, and where gas entrapped beneath a vessel is used as the mantle, the vessel should be "inverted". The U.K. Patent does not disclose any method or apparatus for reducing reverberating noise at a streamer located a substantial horizontal distance from a source due to acoustic waves which have propagated downward from the source, and then reflected upward from the floor (or a subfloor reflecting layer) and downward from the water surface to be detected by the hydrophones of the streamer. Nor does the U.K. Patent disclose use of a mantle of gas as an acoustic lens for refracting acoustic waves reflected from the water surface or floor to reduce the reverberating noise received by the hydrophones.